A multifunctional water-retaining agent suitable for acidic soil and preparation method thereof

Abstract

The present invention discloses a multifunctional water-retaining agent adapted to acidic soil and a preparation method thereof. The multifunctional water-retaining agent is composed of the following raw materials in parts by weight: 9 to 11 parts of a chitosan solution with a concentration of 0.019 to 0.021 g / mL, 9 to 11 parts of a sodium polyphosphate solution with a mass concentration of 0.5 to 1%, 24 to 25 parts of an acrylic acid solution with a neutralization degree of 80 to 85%, and 9 to 11 parts of a guar gum solution with a concentration of 0.019 to 0.021 g / mL, accounting for 0.019 to 0.021 g / mL of the total dry matter weight of the chitosan, guar gum, and acrylic acid. 0.01-0.03% methylenebisacrylamide, 0.25-0.35% of the total dry matter weight of chitosan, guar gum and acrylic acid, and 10-12 parts of clay minerals; the preparation method comprises the following steps: S1, pre-preparation, S2, mixing, and S3, post-treatment; the water-retaining agent prepared by the present invention can not only greatly improve the water absorption and water retention performance of the chitosan water-retaining agent, but also reduce the acidity of acidic soil and improve the soil CEC and fertilizer retention capacity, thereby being a multifunctional soil conditioner for acidic red-yellow soil.

Description

Technical Field

[0001] The invention relates to the technical field of soil admixtures, in particular to a multifunctional water-retaining agent suitable for acidic soil and a preparation method thereof. Background Art

[0002] Water-retaining agents, also known as superabsorbents or super absorbents, are widely used in agriculture, medicine, textiles, construction, and other fields due to their unique three-dimensional network structure and the numerous hydrophilic functional groups (such as carboxyl, amino, and hydroxyl groups) attached to their polymer backbone. They can absorb and store tens to nearly a hundred times more salt water or thousands of times more water. They are also increasingly used in soil improvement. By improving soil structure and water retention, they can retain nutrients, reduce evaporation and underground seepage, effectively alleviating drought and even fertilizing problems. They are important conditioners for regulating soil water, fertilizer, air, and heat conditions. Currently, the predominant water-retaining agents used on the market are synthetic resins, primarily made from acrylic acid and acrylates. These agents boast high water absorption, strong salt tolerance, and excellent composite properties. However, most of these agents are non-biodegradable. Consequently, the use of biodegradable byproducts (such as starch, cellulose, and sodium humate) as monomers to synthesize environmentally friendly water-retaining agents is gaining widespread attention. This type of natural polymer water-retaining agent offers advantages such as widespread availability, low production costs, environmental friendliness, and easy degradation. However, compared to synthetic resin-based water-retaining agents, their water absorption capacity is lower and their production process is more complex. Therefore, the development of environmentally friendly water-retaining agents with simple production processes, high water retention, and diverse functions, while maintaining environmental sustainability, is a key development trend in water-retaining agent production.

[0003] At present, the research and development of most soil water-retaining agents is mainly aimed at the properties of saline-alkali soils in the north. For example, for most super absorbent hydrogels, the pH value for optimal water retention and nutrient retention is about 7. The pH of the red and yellow soils in the south that suffer from seasonal drought is usually in the range of 4-6.5, and they are subject to strong weathering and leaching. The soil has a low CEC, weak fertilizer retention capacity, and prominent acid and lean problems. The good water and heat conditions in southern my country are an excellent environment for crop growth, but seasonal drought and the accompanying acid and lean problems are the main factors limiting crop growth. It is urgent to develop environmentally friendly water-retaining agents with high water absorption performance and low cost under acidic conditions to better solve the various problems of seasonal drought, acid and leanness in acidic red and yellow soils. Therefore, there is an urgent need to develop a multifunctional water-retaining agent that is suitable for acidic red and yellow soils and can increase soil pH, CEC, and saturated water content. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a multifunctional water-retaining agent suitable for acidic soil and a preparation method thereof.

[0005] The technical solution of the present invention is: a multifunctional water-retaining agent adapted to acidic soil, composed of the following raw materials in parts by weight:

[0006] 9 to 11 parts of a chitosan solution with a concentration of 0.019 to 0.021 g / mL, 9 to 11 parts of a sodium polyphosphate solution with a mass concentration of 0.5 to 1%, 24 to 25 parts of an acrylic acid solution with a neutralization degree of 80 to 85%, 9 to 11 parts of a guar gum solution with a concentration of 0.019 to 0.021 g / mL, 0.01 to 0.03% of methylenebisacrylamide in terms of the total dry matter of the chitosan, guar gum and acrylic acid, 0.25 to 0.35% of ammonium persulfate in terms of the total dry matter of the chitosan, guar gum and acrylic acid, and 10 to 12 parts of a clay mineral; the clay mineral is selected from one of saponite, montmorillonite and attapulgite.

[0007] The water-retaining agent prepared using the above ingredients can effectively improve the water-retaining performance of the water-retaining agent; the structure of clay minerals contains porous microscopic channels and voids that can absorb and fix water molecules; the addition of chitosan solution can absorb a large amount of water and form a gel structure. Chitosan has good water absorption and biological activity, and can also promote plant growth and resistance to adversity; by compounding clay minerals and chitosan, a material with better water-retaining performance and stability can be obtained; the composite material promotes the water-retaining agent to more effectively absorb and retain water, and release it to plants or soil during water shortage periods; when clay minerals are added to the water-retaining agent of the present invention, the water absorption performance of the material can be increased. Experimental results show that the addition of clay minerals to the water-retaining agent prepared with chitosan in the present invention significantly improves the water absorption effect of the water-retaining agent, among which the water absorption effects are saponite > montmorillonite > attapulgite. Compared with the chitosan water-retaining agent without clay minerals, the water retention performance increases by 74.4%, 60.9%, and 52.3%, respectively.

[0008] Furthermore, the clay mineral is a clay mineral modified by using 1 to 3 parts of an additive, wherein the additive comprises, by mass percentage, 5 to 10% of sodium alginate, 5 to 10% of nano-TiO2, and the balance of 0.05 to 0.1% of a humic acid solution;

[0009] The modification method comprises: fully mixing the nano-TiO2 and the sodium alginate, sieving the mixture through a sieve with an aperture of 80 to 100 meshes to obtain a composite micropowder; then crushing the clay mineral using a crusher to a particle size of 80 to 100 meshes; and dividing the clay mineral and the humic acid solution into two equal parts.

[0010] The composite micropowder and one portion of the crushed clay mineral are stirred and mixed, and then added to 1 / 2 of the humic acid solution, the temperature is raised to 70-80° C., and stirred for 10-15 minutes to obtain a mixed suspension A, which is then cooled to room temperature at a rate of 3-5° C. / min. The remaining 1 / 2 of the crushed clay mineral and the remaining 1 / 2 of the humic acid solution are added to the mixed suspension A, and then ultrasonically mixed for 20-25 minutes under the conditions of an ultrasonic frequency of 15-20 kHz and an ultrasonic power of 300-400 W to obtain a mixed suspension B; the mixed suspension B is dried in a drying chamber at 60-70° C. for 30-40 minutes and ground into 120-140 meshes to obtain the pre-treated clay mineral;

[0011] Description: Humic acid and sodium alginate in the composite additive have good water retention capacity and can improve the water holding capacity of the soil. Adding a small amount of nano-TiO2 can not only improve the water retention performance of clay minerals, but also reduce the impact of ultraviolet radiation on water retaining agents and soil due to its good absorption and scattering effects, thereby improving the stability of water retention performance.

[0012] Adding clay minerals and humic acid twice helps to avoid the problem of uneven mixing caused by adding too much material at one time. During the first addition, the composite micropowder and crushed clay mineral are mixed with part of the humic acid solution and stirred at a specific temperature. This process can initially mix the minerals and humic acid, while allowing the humic acid to exert its colloidal properties, hydrophilicity and other characteristics, which helps to evenly disperse and suspend the minerals. Subsequently, by controlling the cooling rate to room temperature, the state of the mixed suspension can be further stabilized. Adding the remaining clay minerals and humic acid solution for the second time and mixing them under ultrasonic conditions can further improve the uniformity and stability of the mixing. The mechanical action and thermal effect of the ultrasonic wave can further disperse the particles and improve the uniformity of the mixture. In the preparation method of the composite adjuvant of the present invention, by fully mixing the composite micropowder, humic acid solution and clay mineral, the effect of the clay mineral on the chitosan water-retaining agent can be effectively enhanced, thereby further improving the performance of the water-retaining agent.

[0013] Furthermore, before the composite micropowder is added, the composite micropowder is modified to obtain gelatinized composite micropowder, and the modification method is as follows: the composite micropowder is subjected to microwave treatment at a temperature of 50-60° C., a microwave power of 270-300 W, and normal pressure for 10-12 minutes, and then the temperature is raised to 75-80° C., nitrogen is introduced and the pressure is increased to 0.1-0.15 MPa, and the pH is adjusted to 7-8, and the mixture is stirred for 15-20 minutes to obtain gelatinized composite micropowder;

[0014] Description: Sodium alginate and nano-titanium dioxide powders are heated using microwave treatment. In a nitrogen atmosphere, sodium alginate acts as a carbon source through a carbonization process to form a carbon nanostructure. When carbonized sodium alginate and nano-titanium dioxide coexist, a TiO2 / C nanocomposite material with a hierarchical porous structure can be formed between the two. The TiO2 / C nanocomposite material can be adsorbed on the surface of clay minerals, thereby expanding the particle gaps of the clay minerals and further enhancing the water absorption effect of the clay minerals on the chitosan water-retaining agent.

[0015] Furthermore, the operating parameters of the crusher are: power of 15-20 kW, speed of 200-220 r / min, and crushing time of 50-60 min;

[0016] Note: The above crushing parameters can produce uniform composite additive powder.

[0017] Furthermore, the clay mineral is pretreated; the pretreatment method comprises: spraying a 7-8% H2O2 solution in the clay mineral at a mass ratio of clay mineral to hydrogen peroxide solution of 1.7-2:1-3, heating the clay mineral with steam at a heating temperature of 70-85°C for 3-10 minutes; and then adjusting the moisture content of the clay mineral to 10-15% RH using a dehumidifier;

[0018] Description: Pre-treatment of clay minerals can reduce the presence of crystalline water and organic matter in clay minerals, making the surface of mineral particles purer, thereby increasing its specific surface area; in addition, steam heating can penetrate into the interior of clay minerals more evenly than ordinary heating methods, thereby achieving a more uniform heating effect, and also helps to reduce the damage to the mineral structure caused by rapid temperature changes; then reducing the water content ratio and the moisture in the minerals, making the mineral particles more compact, increasing small gaps and thus increasing its specific surface area, and the hydrophilic groups on the surface of clay minerals are more active, which helps to improve its wettability and dispersibility; thereby playing a role in improving the performance of water retaining agents.

[0019] The present invention also provides a method for preparing a multifunctional water-retaining agent adapted to acidic soil, comprising the following steps:

[0020] S1. Preparation

[0021] S1-1. Preparation of chitosan solution: Weigh chitosan powder according to the concentration and dissolve it in an acidic solution. Seal the solution with a sealing film and ultrasonically treat it at an ultrasonic frequency of 30 to 50 Hz for 3 to 5 minutes. Store the solution at room temperature for 22 to 26 hours to obtain a chitosan solution with a concentration of 19 to 21 g / mL, which is then used for standby use.

[0022] S1-2, preparation of guar gum solution: weigh guar gum according to the concentration and add it to deionized water at 65-75°C, maintain constant temperature, stir until it is completely dissolved into a paste, and set aside;

[0023] S1-3. Preparation of acrylic acid solution: Acrylic acid is measured at the concentration described above and placed in a beaker. A 28-32% NaOH solution is added dropwise at a rate of 3-5 mL / s in an ice-water bath. The mixture is stirred and neutralized to obtain an acrylic acid solution with a neutralization degree of 80-85%, which is then set aside.

[0024] S2, mixed

[0025] The chitosan solution is added to the sodium polyphosphate solution, and the mixture is stirred in a constant temperature water bath at 55-65° C. for 25-35 minutes for prepolymerization. Then, acrylic acid solution and methylenebisacrylamide are added in sequence, and the mixture is stirred at 55-65° C. for 10-20 minutes. The guar gum solution and clay mineral are added to the system, and the temperature is raised to 65-75° C., followed by the addition of ammonium persulfate and stirring until a gel is generated. Nitrogen is continuously introduced during the reaction.

[0026] S3, post-processing

[0027] After the gel is cooled, it is taken out and rinsed with anhydrous ethanol for 2 to 4 times, and finally dried and crushed for later use to obtain a multifunctional water-retaining agent suitable for acidic soil.

[0028] Furthermore, in step S1-1, the acidic solution is glacial acetic acid with a mass concentration of 9 to 10%;

[0029] Description: Chitosan has good solubility in glacial acetic acid and forms a uniform solution. Compared with other acidic solutions, the chitosan solution formed by dissolving chitosan powder in glacial acetic acid solution has better stability and durability. In addition, chitosan powder dissolved in glacial acetic acid solution can promote the formation of soil aggregates, increase soil organic matter, promote microbial activity and provide nutrient sources, thereby improving soil water retention and fertility in many ways, thereby improving soil quality and promoting plant growth and development.

[0030] Furthermore, in step S3, the drying method is to use a spray drying method for primary drying treatment, the primary drying time is 10 to 20 minutes; then the temperature is raised to 75 to 85°C at a rate of 10 to 15°C / min, and finally placed in a constant temperature drying oven at 75 to 85°C for secondary drying treatment, the secondary drying time is 10 to 30 minutes;

[0031] Description: Spray drying has the characteristics of fast heat transfer, rapid water evaporation and short drying time, which can make the gel dry quickly and facilitate subsequent crushing and screening.

[0032] Furthermore, in step S3, the crushing parameters are: crushing pressure of 1 to 1.5 MPa, crushing time of 1 to 3 hours, crushing temperature of 22 to 26° C., and particle size of 40 to 100 μm;

[0033] Note: The crushing effect is better under the above crushing parameters, and the water retention performance of the prepared multifunctional water-retaining agent is better.

[0034] Furthermore, in step S2, the purity of the nitrogen is 99.99-99.999%;

[0035] Note: Maintaining nitrogen purity within the above range can reduce impurities and prevent explosive boiling.

[0036] The beneficial effects of the present invention are:

[0037] (1) When clay minerals are added to the water-retaining agent, the water absorption performance of the material can be increased. The experimental results show that the addition of clay minerals to the water-retaining agent prepared by the present invention on the basis of chitosan greatly improves the water absorption effect of the water-retaining agent, among which the water absorption effect is saponite > montmorillonite > attapulgite. Compared with the chitosan water-retaining agent without adding clay minerals, the water retention performance increases by 74.4%, 60.9%, and 52.3%, respectively.

[0038] (2) The chitosan water-retaining agent prepared with the assistance of clay minerals in the present invention can not only improve the water absorption and water retention properties of the chitosan water-retaining agent, but also has better water retention effect and stability at high temperature. After 20 days at high temperature, the water retention effect of commercial water-retaining agents and unimproved water-retaining agents on the market is basically reduced to 0, while the water supply capacity of the improved water-retaining agent can be extended by one month; and the water-retaining agent prepared by the present invention is less affected by pH; at the same time, it can reduce the acidity of acidic soil and improve the fertilizer retention capacity of the soil.

[0039] (3) The preparation method of the multifunctional water-retaining agent of the present invention is that dissolving chitosan in glacial acetic acid solution causes less damage to the structure of chitosan than dissolving it in other acidic solutions, and can maintain its high molecular weight, which helps to maintain its durability and improvement effect in the soil; the guar gum solution is first gelatinized to form a gelatinized structure of the guar gum solution, forming a uniformly dispersed mixture colloid. The stability of this colloid can effectively improve the preparation efficiency of the water-retaining agent, further improve the stability of the water-retaining performance, and thus improve the improvement effect of the water-retaining agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a surface structure diagram of the chitosan water-retaining agent under SEM in the present invention;

[0041] Figure 2 This is a surface structure diagram of the saponite-modified chitosan water-retaining agent under SEM in the present invention;

[0042] Figure 3 This is a surface structure diagram of the montmorillonite-modified chitosan water-retaining agent under SEM in the present invention;

[0043] Figure 4 FTIR spectra of chitosan water-retaining agent, saponite-modified chitosan water-retaining agent, and montmorillonite-modified chitosan water-retaining agent in the present invention;

[0044] Figure 5 This is a comparison chart of the water retention effects of various water retaining agents after baking at 40℃ for different times;

[0045] Figure 6 It is the water absorption performance of various water-retaining agents when used repeatedly;

[0046] Figure 7 It is the water absorption performance of various water-retaining agents in different pH solutions;

[0047] Figure 8 The water retention effect diagrams of the water-retaining agent of the present invention when applied to Jiangxi red sand soil (A) are shown;

[0048] Figure 9 The water retention effect diagrams of the water-retaining agent of the present invention applied to Anhui red soil (B) are shown;

[0049] Figure 10 The figures are water-retention effect diagrams of the water-retention agent of the present invention applied to Gansu fluvo-aquic soil (C). DETAILED DESCRIPTION

[0050] The present invention will be further described in detail below in conjunction with specific implementation methods to better demonstrate the advantages of the present invention.

[0051] Example 1: A multifunctional water-retaining agent adapted to acidic soil, comprising the following raw materials in parts by weight: 10 parts of a chitosan solution having a concentration of 0.020 g / mL, 10 parts of a sodium polyphosphate solution having a mass concentration of 0.8%, 24 parts of an acrylic acid solution having a neutralization degree of 83%, 10 parts of a guar gum solution having a concentration of 0.020 g / mL, methylenebisacrylamide accounting for 0.02% of the total dry matter of the chitosan, guar gum, and acrylic acid, ammonium persulfate accounting for 0.3% of the total dry matter of the chitosan, guar gum, and acrylic acid, and 11 parts of a clay mineral; the clay mineral being saponite;

[0052] The clay mineral was pretreated by spraying a 7.5% H2O2 solution into the clay mineral at a mass ratio of 1.8:2, heating the clay mineral with steam at 80°C for 5 minutes, and then adjusting the moisture content of the clay mineral to 13% RH using a dehumidifier.

[0053] The preparation method of the chitosan-based water-retaining agent modified with clay minerals mentioned above comprises the following steps:

[0054] S1. Preparation

[0055] S1-1. Preparation of chitosan solution: Chitosan powder was weighed according to the concentration and dissolved in an acidic solution. The mixture was sealed with a sealing film and ultrasonically treated at a frequency of 40 Hz for 4 minutes. The mixture was then stored at room temperature for 24 hours to obtain a chitosan solution with a concentration of 20 mg / mL for later use. In step S1-1, the acidic solution used was glacial acetic acid with a mass concentration of 9.5%.

[0056] S1-2, preparation of guar gum solution: weigh guar gum according to the concentration and add it to 70°C deionized water, maintain constant temperature, stir until it is completely dissolved into a paste, and set aside;

[0057] S1-3. Preparation of acrylic acid solution: Add acrylic acid according to the concentration into a beaker, add 30% NaOH solution dropwise at a rate of 4 mL / s in an ice-water bath, and stir to neutralize to obtain an acrylic acid solution with an 83% neutralization degree, which is set aside;

[0058] S2, mixed

[0059] The chitosan solution was added to the sodium polyphosphate solution and stirred in a constant temperature water bath at 60°C for 30 minutes for prepolymerization. Then, acrylic acid solution and methylene bisacrylamide were added in sequence. After stirring at 60°C for 15 minutes, the guar gum solution and clay mineral were added to the system. After the temperature was raised to 70°C, ammonium persulfate was added and stirred until a gel was formed. Nitrogen gas was continuously introduced during the reaction; the purity of the nitrogen gas was 99.995%.

[0060] S3, post-processing

[0061] After the gel is cooled, it is taken out, rinsed with anhydrous ethanol three times, and finally dried and crushed for later use to obtain a multifunctional water-retaining agent adapted to acidic soil; in step S3, the drying method is to use a spray drying method for primary drying treatment, and the primary drying time is 15 minutes; then the temperature is raised to 80°C at a rate of 13°C / min, and finally placed in a constant temperature drying oven at 80°C for secondary drying treatment, and the secondary drying time is 20 minutes; the crushing parameters are: crushing pressure of 1.2 MPa, crushing time of 2 hours, crushing temperature of 24°C, and particle size of 40 to 100 μm.

[0062] Example 2: Different from Example 1, a clay mineral-modified chitosan-based water-retaining agent is composed of the following raw materials in parts by weight: 11 parts of chitosan solution with a concentration of 0.019 g / mL, 11 parts of sodium polyphosphate solution with a mass concentration of 0.5%, 25 parts of acrylic acid solution with a neutralization degree of 80%, 9 parts of guar gum solution with a mass concentration of 0.019 g / mL, methylenebisacrylamide accounting for 0.01% of the total dry matter of the chitosan, guar gum, and acrylic acid, ammonium persulfate accounting for 0.25% of the total dry matter of the chitosan, guar gum, and acrylic acid, and 10 parts of clay mineral.

[0063] Example 3: Different from Example 1, a clay mineral-modified chitosan-based water-retaining agent is composed of the following raw materials in parts by weight: 9 parts of chitosan solution with a concentration of 0.021 g / mL, 9 parts of sodium polyphosphate solution with a mass concentration of 1%, 24 parts of acrylic acid solution with a neutralization degree of 85%, 11 parts of guar gum solution with a mass concentration of 0.021 g / mL, methylenebisacrylamide accounting for 0.03% of the total dry matter of the chitosan, guar gum, and acrylic acid, ammonium persulfate accounting for 0.35% of the total dry matter of the chitosan, guar gum, and acrylic acid, and 12 parts of clay mineral.

[0064] Example 4: Different from Example 1, the clay mineral is montmorillonite.

[0065] Example 5: Different from Example 1, the clay mineral used is attapulgite.

[0066] Example 6: The pretreatment method is as follows: according to the mass ratio of clay mineral to hydrogen peroxide solution of 1.7:1, a 7% mass concentration of H2O2 solution is sprayed on the clay mineral, and the clay mineral is heated with steam at a heating temperature of 70°C and a heating time of 3 minutes; then the moisture content of the clay mineral is adjusted to 10% RH by a dehumidifier.

[0067] Example 7: The pretreatment method is as follows: according to the mass ratio of clay mineral to hydrogen peroxide solution of 2:3, an H2O2 solution with a mass concentration of 8% is sprayed on the clay mineral, and the clay mineral is heated with steam at a heating temperature of 85°C and a heating time of 10 minutes; then the moisture content of the clay mineral is adjusted to 15% RH by a dehumidifier.

[0068] Example 8: The difference from Example 1 is that S1-1, preparation of chitosan solution: chitosan powder is weighed according to the concentration and dissolved in an acidic solution, sealed with a sealing film, ultrasonically treated at an ultrasonic frequency of 30 Hz for 5 minutes, and stored at room temperature for 22 hours to obtain a chitosan solution with a concentration of 19 g / mL, which is set aside.

[0069] Example 9: The difference from Example 1 is that S1-1, preparation of chitosan solution: chitosan powder is weighed according to the concentration and dissolved in acidic solution, sealed with sealing film, ultrasonically treated at an ultrasonic frequency of 50 Hz for 3 minutes, and stored at room temperature for 26 hours to obtain a chitosan solution with a concentration of 21 g / mL for use.

[0070] Example 10: The difference from Example 1 is that S1-3, preparation of acrylic acid solution: acrylic acid is measured in a beaker according to the concentration, and a 28% mass concentration NaOH solution is added dropwise at a rate of 3 mL / s under ice-water bath conditions, and stirred for neutralization to obtain an acrylic acid solution with an 80% neutralization degree, which is set aside.

[0071] Example 11: The difference from Example 1 is that S1-3, preparation of acrylic acid solution: acrylic acid is measured in a beaker according to the concentration, and a NaOH solution with a mass concentration of 32% is added dropwise at a rate of 5 mL / s under ice-water bath conditions, and stirred for neutralization to obtain an acrylic acid solution with an 85% neutralization degree, which is set aside.

[0072] Example 12: Different from Example 1, in step S2, the chitosan solution is added to the sodium polyphosphate solution, and the solution is stirred in a constant temperature water bath at 55°C for 35 minutes for prepolymerization, and then acrylic acid solution and methylenebisacrylamide are added thereto in sequence. After stirring at 55°C for 20 minutes, the guar gum solution and clay mineral are added to the system. After the temperature is raised to 65°C, ammonium persulfate is continued to be added and stirred until a gel is produced.

[0073] Example 13: Different from Example 1, in step S2, the chitosan solution is added to the sodium polyphosphate solution, and the solution is stirred in a constant temperature water bath at 65°C for 25 minutes for prepolymerization, and then acrylic acid solution and methylenebisacrylamide are added thereto in sequence. After stirring at 65°C for 10 minutes, the guar gum solution and clay mineral are added to the system. After the temperature is raised to 75°C, ammonium persulfate is continued to be added and stirred until a gel is produced.

[0074] Example 14: Different from Example 1, in step S3, the drying method is to use spray drying method for the initial drying treatment, and the initial drying time is 10 minutes; then the temperature is raised to 75°C at a rate of 10°C / min, and finally placed in a constant temperature drying oven at 75°C for secondary drying treatment, and the secondary drying time is 10 minutes.

[0075] Example 15: Different from Example 1, in step S3, the drying method is to use spray drying method for the initial drying treatment, and the initial drying time is 20 minutes; then the temperature is raised to 85°C at a rate of 15°C / min, and finally placed in a constant temperature drying oven at 85°C for secondary drying treatment, and the secondary drying time is 30 minutes.

[0076] Example 16: Different from Example 1, in step S3, the crushing parameters are: crushing pressure of 1 MPa, crushing time of 1 h, crushing temperature of 22°C, and particle size of 40-100 μm.

[0077] Example 17: Different from Example 1, in step S3, the crushing parameters are: crushing pressure of 1.5 MPa, crushing time of 3 h, crushing temperature of 26°C, and particle size of 40-100 μm.

[0078] Example 18: Different from Example 1, the clay mineral is a clay mineral modified with two parts of additives, wherein the additives, calculated by mass percentage, include: 8% sodium alginate, 8% nano-TiO2, and the balance 0.07% humic acid solution;

[0079] The modification method is as follows: nano-TiO2 and sodium alginate are mixed and sieved through a sieve with an aperture of 80-100 mesh to obtain a composite micropowder; the clay mineral is then crushed to a particle size of 80-100 mesh using a crusher; the clay mineral and humic acid solution are divided into two equal parts, and the operating parameters of the crusher are: power of 18 kW, speed of 210 r / min, and crushing time of 55 min;

[0080] The composite micropowder and one portion of the crushed clay mineral were stirred and mixed, and then added to 1 / 2 of the humic acid solution, heated to 75°C, stirred for 12 minutes to obtain a mixed suspension A, cooled to room temperature at a rate of 4°C / min, and then added to the mixed suspension A with the remaining 1 / 2 of the crushed clay mineral and the remaining 1 / 2 of the humic acid solution. The mixture was then ultrasonically mixed for 23 minutes under the conditions of an ultrasonic frequency of 18 kHz and an ultrasonic power of 350 W to obtain a mixed suspension B. The mixed suspension B was dried in a drying chamber at 65°C for 35 minutes and ground to a particle size of 120 to 140 meshes to obtain the modified clay mineral.

[0081] The composite micropowder was modified to obtain gelatinized composite micropowder. The modification method was as follows: the composite micropowder was microwave-treated at a temperature of 55°C, a microwave power of 285W, and normal pressure for 11 minutes, then the temperature was raised to 78°C, nitrogen was introduced and the pressure was increased to 0.13 MPa, the pH was adjusted to 7.5, and the gelatinized composite micropowder was obtained after stirring for 18 minutes.

[0082] Example 19: Different from Example 18, the clay mineral is a clay mineral modified by using 1 part of an additive, and the additive comprises, by mass percentage, 5% sodium alginate, 5% nano-TiO2 and the remainder having a mass concentration of 0.05% humic acid solution.

[0083] Example 20: Different from Example 18, the clay mineral is a clay mineral modified by using 3 parts of additives, and the additives include, by mass percentage, 10% sodium alginate, 10% nano-TiO2 and the remainder of humic acid solution with a mass concentration of 0.1%.

[0084] Example 21: Different from Example 18, the modification method is as follows: nano-TiO2 and sodium alginate are fully mixed, sieved with a sieve with an aperture of 80 to 100 mesh to obtain a composite micropowder; then the clay mineral is crushed to a particle size of 80 to 100 mesh using a crusher; the clay mineral and the humic acid solution are divided into two equal parts, the composite micropowder and one of the crushed clay minerals are stirred and mixed, and then added to 1 / 2 of the humic acid solution, heated to 70°C, stirred for 15 minutes to obtain a mixed suspension A, cooled to room temperature at a rate of 3°C / min, and then the remaining 1 / 2 of the crushed clay mineral and the remaining 1 / 2 of the humic acid solution are added to the mixed suspension A, and then ultrasonically mixed for 25 minutes under the conditions of an ultrasonic frequency of 15 kHz and an ultrasonic power of 300 W to obtain a mixed suspension B; the mixed suspension B is dried in a drying chamber at 60°C for 40 minutes and ground to a particle size of 120 to 140 mesh to obtain the modified clay mineral.

[0085] Example 22: Different from Example 18, the modification method is as follows: nano-TiO2 and sodium alginate are fully mixed, and sieved with a sieve with an aperture of 80-100 mesh to obtain a composite micropowder; then the clay mineral is crushed with a crusher to a particle size of 80-100 mesh; the clay mineral and humic acid solution are divided into two equal parts,

[0086] The composite micropowder and one portion of the crushed clay mineral were stirred and mixed, and then added to 1 / 2 of the humic acid solution. The temperature was raised to 80°C and stirred for 10 minutes to obtain a mixed suspension A. The temperature was cooled to room temperature at a rate of 5°C / min. The remaining 1 / 2 of the crushed clay mineral and the remaining 1 / 2 of the humic acid solution were added to the mixed suspension A. The mixture was then ultrasonically mixed for 20 minutes under the conditions of an ultrasonic frequency of 20 kHz and an ultrasonic power of 400 W to obtain a mixed suspension B. The mixed suspension B was dried in a drying chamber at 70°C for 30 minutes and ground to a particle size of 120 to 140 meshes to obtain the modified clay mineral.

[0087] Example 23: Different from Example 18, the operating parameters of the crusher are: power of 15kw, speed of 200r / min, and crushing time of 60min.

[0088] Example 24: Different from Example 18, the operating parameters of the crusher are: power of 20kw, speed of 220r / min, and crushing time of 50min.

[0089] Example 25: Different from Example 18, the composite micropowder is modified before being added to obtain gelatinized composite micropowder. The modification method is: the composite micropowder is microwave-treated for 12 minutes at a temperature of 50°C, a microwave power of 270W, and normal pressure, and then the temperature is raised to 75°C, nitrogen is introduced and the pressure is increased to 0.1MPa, and the pH is adjusted to 7. After stirring for 15 minutes, gelatinized composite micropowder is obtained.

[0090] Example 26: Different from Example 18, the composite micropowder is modified before being added to obtain gelatinized composite micropowder. The modification method is: the composite micropowder is microwave-treated at a temperature of 60°C, a microwave power of 300W, and normal pressure for 10 minutes, and then the temperature is raised to 80°C, nitrogen is introduced and the pressure is increased to 0.15MPa, and the pH is adjusted to 8. After stirring for 20 minutes, gelatinized composite micropowder is obtained.

[0091] Control Example

[0092] Comparative Example 1: Different from Example 1, no clay mineral is added to the composite additive.

[0093] Comparative Example 2: Different from Example 1, the clay mineral is not pretreated.

[0094] Control Example 3: Different from Example 18, the preparation method of the auxiliary agent is to directly mix the ingredients.

[0095] Control Example 4: The difference from Example 1 is that S1-3, preparation of acrylic acid solution: acrylic acid is measured in a beaker according to the proportion, and a 30% mass concentration NaOH solution is added dropwise at a rate of 4 mL / s under ice-water bath conditions, and stirred for neutralization to obtain a 90% neutralized acrylic acid solution for use.

[0096] Comparative Example 5: Different from Example 1, in step S3, the product is dried in a constant temperature drying oven at 80°C for 20 minutes.

[0097] Comparative Example 6: Different from Example 1, in step S3, anhydrous ethanol is not used to rinse the gel.

[0098] Experimental example:

[0099] 1. Analysis of the attached figures

[0100] The morphology of the synthesized water-retaining agent was analyzed by scanning electron microscopy (SEM) at a magnification of 5000. Figures 1 to 3The results show that the surface of the chitosan water-retaining agent is uneven. The addition of clay minerals causes more particles and wrinkles to appear on the surface of the water-retaining agent, increasing the specific surface area of ​​the water-retaining agent and expanding the network space of the water-retaining agent's cross-linked gel, providing more active water absorption points. At the same time, more pores appear on the surface, providing channels for water molecules to enter the water-retaining agent's network structure. The combined effect of these two factors significantly improves the water absorption efficiency of the water-retaining agent.

[0101] The infrared spectrum analysis results of synthetic water retaining agent are as follows Figure 4 As shown. FTIR spectrum at 3377cm -1 The broad peak at 2926 cm-1 is attributed to the OH stretching vibration. -1 The peak at 1564 cm is the C–CH2–C dynamic peak. -1 The N–H absorption vibration peak appears at 1454 cm, indicating that chitosan has successfully reacted with the crosslinker (methylene bisacrylamide) and initiator (ammonium persulfate). -1 The absorption vibration peak of -CH2 is at 3377cm, indicating that guar gum also reacts with the former. -1 The peak at 3397 cm -1 3382cm -1 It shows that O–H forms new bonds with Al, Fe, etc., indicating that saponite and montmorillonite participate in the reaction of water retaining agent synthesis.

[0102] Depend on Figure 5 The water-retaining agent prepared by this method has a better water absorption effect than commercial water-retaining agents. After 20 days, the water retention effect of commercial water-retaining agents and unmodified water-retaining agents is basically reduced to 0, while the modified water-retaining agent still retains 30-40% of water after one month of high temperature.

[0103] Depend on Figure 6 It can be seen that when the water-retaining agent absorbs water, is dried, and then absorbs water again, and its effect of multiple uses is measured, the water absorption effect of the improved water-retaining agent is better than that of commercial water-retaining agents and unimproved water-retaining agents when used for the first and second time. However, its effect gradually decreases with the increase in the number of uses, and the water absorption effect is completely lost when it is used for the fourth time.

[0104] Depend on Figure 7It can be seen that within the pH range of 3 to 8, there was no significant correlation between pH and the water absorption properties of commercial and chitosan water-retaining agents (r = -0.02, P = 0.98; r = -0.50, P = 0.11). However, the saponite and montmorillonite water-retaining agents maintained good water-retention properties between pH 4 and 7. When the solution pH was 3 or 8, the water absorption properties of the water-retaining agents showed a downward trend, indicating that the water-retaining agents were stable to changes in the solution pH from acidic to neutral, making them very suitable for acidic to neutral soils. Under extremely acidic conditions, the protonation of the carboxylate anions on the surface of the water-retaining agent reduced the repulsive forces between the anions, thus further cross-linking the hydrogel network would limit its swelling capacity. Under alkaline conditions, the excess cations weakened the electrostatic repulsion by shielding the charge of the carboxylate anions. At the same time, the water absorption performance of the water retaining agent in the electrolyte solution is significantly reduced. The reason is that in a salty environment, the negative charge of the water retaining agent is shielded by the salt cations, resulting in a decrease in the mutual repulsion. Figure 7 It can be found that in the electrolyte solution, montmorillonite water retaining agent has the best effect, which may be related to the high negative charge of montmorillonite and its high buffering performance.

[0105] For the water-retaining agents prepared in Examples 1-26 and Comparative Examples 1-6, 5 g of each was weighed and placed in a nylon mesh bag. The bag was immersed in distilled water until water absorption equilibrium was reached. Excess distilled water was then slowly poured out. Two cubes of water-retaining agent with a side length of 1 cm were separated using a cotton thread. Each pair of water-retaining agents formed a group. The water-absorbed mass m1 of the cubes and the average dry matter mass m2 were weighed. The water-retaining effectiveness of the water-retaining agents prepared using different preparation methods was calculated using the formula R = (m1 - m2) / m2.

[0106] 2. Explore the effects of clay mineral addition, additive addition, and additive preparation methods on the water retention performance of water retaining agents

[0107] Table 1 Performance measurement of water-retaining agents prepared in Examples 1 to 15 and Comparative Examples 1 to 4

[0108]

[0109] Conclusion: From Examples 1 and 4-5, it can be seen that the effect of clay minerals on improving water retention is in the order of saponite > montmorillonite > attapulgite; in Control Example 1, no clay minerals were added, and the water retention effect was significantly lower than that of Examples 1 and 4-5; therefore, it can be concluded that the addition of clay minerals to the water-retaining agent in the present invention can effectively improve the water retention effect of the water-retaining agent;

[0110] Comparison of the data of Examples 1, 6 to 7 and Control Example 2 shows that the lack of pre-treatment of the clay mineral in Control Example 2 results in a lower water retention performance of the water retaining agent prepared in Control Example 3. This is mainly because the addition of hydrogen peroxide solution and heating can effectively reduce the crystallization water and organic matter in the clay mineral, making the surface of the mineral particles purer, thereby increasing its specific surface area, and further enhancing the effect of the clay mineral on the chitosan water retaining agent, thereby improving the water retention performance of the water retaining agent; the comparison of Examples 1, 2 to 5 and 18 to 26 shows that the water retaining agent prepared in Control Example 3 has a lower water retention performance. It can be seen that the modification of clay minerals with additives and the addition of the modified clay minerals to the water retaining agent significantly improved the water retaining effect of the water retaining agent. Example 18 is the optimal state. However, from the comparison of Example 18, Example 21 to Example 22 and Control Example 3, it can be seen that the water retaining effect of Control Example 3 is significantly lower. This is because the addition of clay minerals and humic acid in two steps by this method helps to avoid the problem of uneven mixing or excessive local concentration caused by adding too much material at one time, which is more conducive to improving the effect of clay minerals on the chitosan water retaining agent and further improving the water retaining performance of the water retaining agent.

[0111] 3. Investigate the effect of acrylic acid neutralization degree on water retention agent performance

[0112] Table 2 Performance measurement of water-retaining agents prepared in Example 1, Examples 10-11 and Comparative Example 4

[0113]

[0114] Conclusion: From the comparison of the data of Example 1, Examples 10-11, and Control Example 4, it can be seen that as the percentage of neutralization of acrylic acid increases, the water absorption performance of the saponite and montmorillonite modified water-retaining agents first increases and then decreases. At 85% and 80%, the water absorption effect is the best, reaching 833.0g / g and 866.5g / g respectively. When the neutralization degree reaches 90%, the water absorption effect of the water-retaining agents decreases significantly, by 35.4% and 58.2%. This is mainly because when NaOH partially neutralizes acrylic acid, NaOH converts the -COOH group into -COO-Na + , in which —COO fixed on the molecular chain - The molecular network structure expands through the action of electrostatic repulsion. In order to maintain electrical neutrality, Na + The concentration difference between the inside and outside of the resin network generates osmotic pressure, which allows water molecules to penetrate and absorb a large amount of water. However, as the degree of neutralization increases, the adjacent -COO - The repulsive force between them increases, the hydrogen bond force between water molecules and ions and the repulsive force between adjacent hydrogen bonds increase, resulting in the inhibition of the expansion of the water retaining agent molecular network, resulting in a decrease in reaction rate and reduced water absorption performance.

[0115] 4. Investigate the effect of water retaining agent preparation temperature on its performance

[0116] Table 3 Performance measurement of water-retaining agents prepared in Example 1, Examples 8-9, 12-17 and Comparative Examples 5-6

[0117]

[0118]

[0119] Conclusion: From the data in Table 3 of Example 1, Examples 14-15, and Comparative Example 6, it can be seen that the drying effect of the water-retaining agent prepared in Comparative Example 6 by not using a gradient temperature drying method cannot ensure uniform heating inside and outside the gel and reduce the thermal stress caused by the temperature difference, thereby deteriorating the stability of the water-retaining performance and causing a significant decrease in the water-retaining effect in Comparative Example 6. In Comparative Example 5, due to the lack of anhydrous ethanol soaking operation, both the specific surface area and adsorption capacity are reduced, resulting in a poor improvement effect of the water-retaining agent.

[0120] 5. Application effect of water-retaining agent in Jiangxi red sandy soil (A), Anhui red soil (B), Gansu fluvo-aquic soil (C)

[0121] The wilting coefficient refers to the soil moisture content at which plants begin to wilt permanently. It's worth noting that the wilting coefficient varies significantly between soils. For wheat, a soil moisture content above 10% is sufficient to prevent permanent wilting. Therefore, using wheat as an example, we analyzed the effects of different water-retaining agents on wilting when the soil moisture content drops to 10%.

[0122] Soil from the surface layer (0-20cm) of farmland in Anhui red soil, Jiangxi red sandy soil and Gansu tidal soil was collected respectively, and the fresh soil was air-dried and ground to pass through a 60-mesh sieve. 100g of soil samples were weighed and placed in 4 plastic cups with holes drilled at the bottom, and two groups were set up in parallel. At the same time, 0.15g of chitosan water-retaining agent, saponite-modified water-retaining agent, montmorillonite-modified water-retaining agent and commercially available water-retaining agent were weighed at an addition amount of 0.15%. They were applied to the soil samples respectively and stirred, and a group of single soil without any water-retaining agent was set up at the same time. After a 300-mesh filter was placed at the bottom of the plastic cup, sufficient deionized water was added for leaching. After the soil samples reached saturated water content, all soil samples were placed in a 40°C constant temperature incubator for culture. At the same time, use a soil sampler to collect soil at 12h, 24h, 36h, 48h, 60h, 72h, and 96h, place it in a plastic cup, and weigh the net weight of the plastic cup and the weight after adding the soil sample, and record them as m0 and m1 respectively. Then put the collected soil into an 80℃ oven, wait for the moisture to be completely dried, and weigh it and record it as m2. The formula for calculating the soil moisture content at each time period is:

[0123] Depend on Figure 8 、 Figure 9 、 Figure 10 It can be seen that the application of water-retaining agents can significantly increase the saturated moisture content of the soil. At the same time, the drought resistance of soils added with modified water-retaining agents (saponite and montmorillonite) is higher than that of soils treated with chitosan water-retaining agents and commercial water-retaining agents. Under extremely high temperature conditions (40°C), the moisture content of red sandy soil decreased to 10% after 37.2 hours, 68.9 hours, 93.7 hours, 102.5 hours, and 124.7 hours of treatment with commercial water-retaining agents, chitosan water-retaining agents, saponite-modified water-retaining agents, and montmorillonite-modified water-retaining agents, respectively. Saponite and montmorillonite treatments extended the water retention time of chitosan water-retaining agents by 9.3% and 33.1%, respectively. Under extremely high temperature (40°C), red soil treated with a commercial water-retaining agent, chitosan water-retaining agent, saponite-modified water-retaining agent, and montmorillonite-modified water-retaining agent reduced its moisture content to 10% after 29.3 hours, 52.4 hours, 61.7 hours, 100.1 hours, and 99.6 hours, respectively. Saponite and montmorillonite treatments extended the water retention time of the chitosan water-retaining agent by 62.2% and 61.4%, respectively. Under extremely high temperature (40°C), fluvo-aquic soil treated with a commercial water-retaining agent, chitosan water-retaining agent, saponite-modified water-retaining agent, and montmorillonite-modified water-retaining agent reduced its moisture content to 10% after 30.1 hours, 38.3 hours, 40.4 hours, 50.7 hours, and 54.6 hours, respectively. Saponite and montmorillonite treatments extended the water retention time of the chitosan water-retaining agent by 25.5% and 35.1%, respectively. This is mainly due to the presence of hydrophilic functional groups in the water-retaining agent. The highly cross-linked water-retaining agent has strong adsorption and complexing capabilities, allowing water to easily enter the internal polymer network and form a water-blocking layer with soil particles. This process prevents the evaporation of water on the soil surface, thereby improving the soil's water retention capacity. Adding saponite and montmorillonite can significantly enhance the role of chitosan water-retaining agent in alleviating soil drought.

[0124] 6. Effects of water-retaining agents on pH and CEC of Jiangxi red sandy soil (A), Anhui red soil (B), and Gansu fluvo-aquic soil (C)

[0125] After the incubation experiment, the soil samples, after adding the water-retaining agent, were air-dried and ground to measure basic soil properties. Soil pH was measured using a pH meter (Orion 3Star, Thermo Fisher Scientific, USA) at a soil-water ratio of 1:2.5 (w:v). The CEC of the soil was also measured using the ammonium acetate method. The results are shown in the table below:

[0126] Table 4 Effect of water retaining agent on pH and CEC of various soils

[0127]

[0128] Studies have shown that the water-retaining agent prepared using this method can improve the basic properties of soil after application. As shown in Table 4, compared with the control soil without water-retaining agent, the pH of the soil after application of water-retaining agent increased significantly, especially in acidic soils. This may be related to the higher alkalinity of the water-retaining agent. The alkalinity of the water-retaining agent was measured to be 6.89 mol / kg for the commercial water-retaining agent; 5.73 mol / kg for the control chitosan water-retaining agent; 5.31 mol / kg for the saponite-modified chitosan water-retaining agent; and 4.24 mol / kg for the montmorillonite-modified chitosan water-retaining agent. At the same time, the application of the water-retaining agent prepared by this method also significantly increased the CEC of the soil. This is because the water-retaining agent contains a large amount of organic matter and has a large number of organic functional groups on its surface, such as carboxyl, hydroxyl, and amide groups. These functional groups carry a large amount of negative charge, and when applied to the soil, they increase the soil CEC. Therefore, the chitosan water-retaining agent synthesized by this invention can not only alleviate the degree of soil drought, but also has a certain effect on improving the acidity of acidic soil and enhancing fertility.

Claims

1. A multifunctional water-retaining agent adapted to acidic soil, characterized in that: It is composed of the following raw materials in parts by weight: 9-11 parts of a chitosan solution with a concentration of 0.019-0.021 g / mL, 9-11 parts of a sodium polyphosphate solution with a mass concentration of 0.5-1%, 24-25 parts of an acrylic acid solution with a neutralization degree of 80-85%, 9-11 parts of a guar gum solution with a concentration of 0.019-0.021 g / mL, 0.01-0.03% of the total dry weight of the chitosan, guar gum, and acrylic acid by weight of methylenebisacrylamide, 0.25-0.35% of the total dry weight of the chitosan, guar gum, and acrylic acid by weight of ammonium persulfate, and 10-12 parts of a clay mineral; the clay mineral is selected from one of saponite, montmorillonite, and attapulgite; The preparation method of the multifunctional water-retaining agent comprises the following steps: S1. Preparation S1-1. Preparation of chitosan solution: Weigh chitosan powder according to the concentration and dissolve it in an acidic solution. Seal the solution with a sealing film and ultrasonicate it at a frequency of 30-50 Hz for 3-5 minutes. Store the solution at room temperature for 22-26 hours to obtain a chitosan solution with a concentration of 0.019-0.021 g / mL, which is then used for standby use. S1-2, preparation of guar gum solution: weigh guar gum according to the concentration and add it to deionized water at 65-75°C, maintain constant temperature, stir until it is completely dissolved into a paste, and set aside; S1-3. Preparation of acrylic acid solution: Acrylic acid is measured at the concentration described above and placed in a beaker. NaOH solution having a mass concentration of 28-32% is added dropwise at a rate of 3-5 mL / s in an ice-water bath. Stir and neutralize to obtain an acrylic acid solution having a neutralization degree of 80-85%, which is then set aside. S2, mixed The chitosan solution is added to the sodium polyphosphate solution, and the mixture is stirred in a constant temperature water bath at 55-65° C. for 25-35 minutes for prepolymerization. Then, acrylic acid solution and methylenebisacrylamide are added in sequence, and the mixture is stirred at 55-65° C. for 10-20 minutes. The guar gum solution and clay mineral are then added to the system. After the temperature is raised to 65-75° C., ammonium persulfate is continuously added and stirred until a gel is generated. Nitrogen is continuously introduced during the reaction. S3, post-processing After the gel is cooled, it is taken out and rinsed with anhydrous ethanol 2 to 4 times. Finally, it is dried and crushed for later use to obtain a multifunctional water-retaining agent suitable for acidic soil. The clay mineral is pretreated; the pretreatment method comprises: spraying a 7-8% H2O2 solution in the clay mineral at a mass ratio of clay mineral to hydrogen peroxide solution of 1.7-2:1-3, heating the clay mineral with steam at a heating temperature of 70-85°C for 3-10 minutes; and then adjusting the moisture content of the clay mineral to 10-15%RH in a dehumidifier; In step S3, the drying method is to use a spray drying method for primary drying treatment, and the primary drying time is 10-20 minutes; then the temperature is raised to 75-85°C at a rate of 10-15°C / min, and finally placed in a constant temperature drying oven at 75-85°C for secondary drying treatment, and the secondary drying time is 10-30 minutes.

2. The multifunctional water-retaining agent adapted to acidic soil according to claim 1, characterized in that: The clay mineral is a clay mineral modified by using 1 to 3 parts of an additive, wherein the additive comprises, by mass percentage, 5 to 10% of sodium alginate, 5 to 10% of nano-TiO2, and the balance of 0.05 to 0.1% of a humic acid solution; The modification method comprises: fully mixing the nano-TiO2 and the sodium alginate, sieving the mixture through a sieve with an aperture of 80-100 mesh to obtain a composite micropowder; then crushing the clay mineral using a crusher to a particle size of 80-100 mesh; and dividing the clay mineral and the humic acid solution into two equal parts. The composite micropowder and one portion of the crushed clay mineral were stirred and mixed, and then added to 1 / 2 of the humic acid solution. The temperature was raised to 70-80°C and stirred for 10-15 minutes to obtain a mixed suspension A. The temperature was lowered to room temperature at a rate of 3-5°C / min. The remaining 1 / 2 of the crushed clay mineral and the remaining 1 / 2 of the humic acid solution were added to the mixed suspension A. The mixture was then ultrasonically mixed for 20-25 minutes under the conditions of an ultrasonic frequency of 15-20 kHz and an ultrasonic power of 300-400 W to obtain a mixed suspension B. The mixed suspension B was dried in a drying chamber at 60-70°C for 30-40 minutes and ground to a particle size of 120-140 meshes to obtain the modified clay mineral.

3. The multifunctional water-retaining agent adapted to acidic soil according to claim 2, characterized in that: Before the composite micropowder is added, the composite micropowder is modified to obtain gelatinized composite micropowder. The modification method is as follows: the composite micropowder is subjected to microwave treatment for 10 to 12 minutes at a temperature of 50 to 60° C., a microwave power of 270 to 300 W, and normal pressure, and then the temperature is raised to 75 to 80° C., nitrogen is introduced and the pressure is increased to 0.1 to 0.15 MPa, the pH is adjusted to 7 to 8, and the mixture is stirred for 15 to 20 minutes to obtain gelatinized composite micropowder.

4. The multifunctional water-retaining agent adapted to acidic soil according to claim 2, characterized in that: The working parameters of the crusher are: power of 15~20kw, rotation speed of 200~220r / min, and crushing time of 50~60min.

5. The multifunctional water-retaining agent adapted to acidic soil according to claim 1, characterized in that: In step S1-1, the acidic solution is glacial acetic acid with a mass concentration of 9-10%.

6. The multifunctional water-retaining agent adapted to acidic soil according to claim 1, characterized in that: In step S3, the crushing parameters are: crushing pressure of 1-1.5 MPa, crushing time of 1-3 hours, crushing temperature of 22-26° C., and particle size of 40-100 μm.

7. The multifunctional water-retaining agent adapted to acidic soil according to claim 1, characterized in that: In step S2, the purity of the nitrogen is 99.99-99.999%.

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